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Cannabis use can raise complex issues for Parkinson's disease patients, including possible adverse effects, toxicity, and drug-drug interactions.

Few studies have shown the benefits or harms of medical cannabis in Parkinson's disease, said Benzi Kluger, MD, MS, of the University of Rochester Medical Center in New York.

Four small randomized trials of cannabinoids have been conducted in Parkinson's patients with mixed results. "These trials were small, short, and had other shortcomings that make it safer to call them inconclusive rather than negative," Kluger said.

"We do know of some side effects -- such as hypotension, apathy, and confusion -- but do not know if there are any long-term effects," he added.

But knowledge about the basic science of cannabinoids and the endocannabinoid system is expanding, Kluger noted. "The literature strongly supports a role for the endocannabinoid system in normal movement and a potential role in many movement disorders."

"There is in animal models and uncontrolled case series in people to support the therapeutic potential of cannabinoids in Parkinson's disease, but we currently have no randomized clinical trials supporting their efficacy for motor or non-motor symptoms in people living with Parkinson's," he said.

In 2014, an American Academy of Neurology (AAN) guideline committee concluded that for treating Parkinson's patients with dyskinesia.

More recently, the AAN published a , supporting academic research and Drug Enforcement Administration rescheduling to make studies easier to conduct. The academy does not support legalizing marijuana because more research about safety and efficacy is needed, but recognizes that medical marijuana may be useful for some neurologic conditions.

In its most recent , the Movement Disorders Society also noted that cannabis-based therapies increasingly were being explored by patients for both motor and non-motor symptoms, but few randomized trials fulfilled evidence-based medicine criteria. "There is a clear need for high-quality randomized controlled trials to evaluate efficacy and, just as important, safety in Parkinson's disease," the society noted.

Cannabis Use Common in Parkinson's

Nearly one in four people with Parkinson's disease have used cannabis recently, according to patient survey data.

The survey findings, presented at the and published on the preprint server , showed that 24.5% of Parkinson's patients reported using cannabis in the past 6 months.

A total of 1,064 Parkinson's patients answered the online survey in January 2020, said James Beck, PhD, chief scientific officer of the Parkinson's Foundation in New York City, and co-authors. Survey respondents had a mean age of 71 and most were men.

Cannabis users were more likely to report insufficient control of their non-motor Parkinson's symptoms with prescription medications than non-users were (P<0.005). They used cannabis to treat symptoms like anxiety (45.5%), pain (44.0%), and sleep disorders (44.0%).

However, 23.0% also reported that they had stopped using cannabis in the previous 6 months, largely due to a lack of symptom improvement (35.5%). And of the survey respondents who did not use cannabis, most said it was because there was a lack of scientific evidence supporting its efficacy (59.9%).

"We found that a majority of people living with Parkinson's disease and using cannabis were doing so without having been provided information or provider recommendations," the researchers noted. "This was not entirely surprising; a previous study found a significant cannabis knowledge gap among specialized Parkinson's disease clinicians that paralleled our observed knowledge gap in people with Parkinson's disease."

A New Consensus Statement

To help address this knowledge gap, the Parkinson's Foundation convened researchers, physicians, and other experts to create the first , issued in May 2020.

"The goals of the consensus statement are to better guide people with Parkinson's and their physicians to help ensure the safe use of medical cannabis for Parkinson's disease symptoms," Beck said.

"We understand from the Parkinson's disease community that this is a topic they want to know more about, but there is very limited information available they could trust," he added. "We hope to fill a gap and better understand the implications of medical cannabis use for Parkinson's disease symptoms overall."

The guidance outlines cannabis products, evidence to date, benefits of cannabis, adverse effects, and safety issues. The guidance counsels Parkinson's patients who want to use medical cannabis to:

• Discuss any use of cannabinoid products and possible drug-drug interactions with physicians

• Start at low doses and increase gradually

• Be aware of potential side effects, especially dizziness, balance problems, worsening motivation, dry mouth, and impaired cognition

The consensus statement is not an endorsement of cannabis, Beck emphasized.

"We cannot endorse the use of medical cannabis until we have more reliable research studies and a better understanding of its impact on people with Parkinson's," he said. "More research on medical cannabis is needed to determine whether its use can have positive or adverse effects for Parkinson's disease symptoms."

"There is much work to be done before we can confidently recommend cannabinoids for any symptom of Parkinson's disease," Kluger noted. "While there are animal studies suggesting neuroprotective effects, we have no evidence in people to recommend cannabinoids for this purpose."

Nonetheless, many people with Parkinson's disease want to try cannabis. Those patients need to involve their physician, be cautious, and "stick with a single dispensary or product, as cannabis is not regulated and labeling can be inaccurate," Kluger said.

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